Semiconductor devices are used in products as extensive as cell phones, radios, and televisions. The semiconductor devices include integrated circuits that are connected by conductive wires embedded in insulating material.
With the reducing of semiconductor device size and the use of low dielectric constant (low k) interlayer dielectric (ILD) insulating materials, obtaining reliable semiconductor devices is becoming more and more challenging. In particular, reliability problems occur at interfaces of the copper (Cu) wires and low k ILD material in the form of leakage, electromigration, stress migration, break down voltage, and time dependent dielectric breakdown (TDDB), etc.
Cu easily diffuses into silicon (Si) and causes degradation of the dielectric material. Cu is also susceptible to oxidation and corrosion. Therefore, a capping layer of a material such as silicon nitride (SiN) or silicon carbide (SiC) is placed on the Cu surface as passivation layer to prevent Cu oxidation and Cu migration into the ILD. However, the Cu/capping layer interface is still one of the major failure paths due to the weak adhesion between Cu and the capping layer.
The dielectric layers are subjected to surface contamination in the manufacturing process (e.g., Cu chemical mechanical polishing (CMP) or electroless plating of a metallic capping layer, such as cobalt tungsten phosphorous (CoWP)). These contaminants are charged and mobile, especially under stress (high temperature and electric field). The mobility of these contaminants cause high leakage currents, and may cause damage to the dielectric materials when they move along the interface.
Low k materials (especially porous low k materials) are less dense than silicon oxide (SiO2) dielectric materials, and have weaker mechanical properties; i.e., the chemical bonds are easier to break.
The damage that is induced by wet chemical or oxidative plasma clean processes are manifested in increasing dangling bond density, transformation of terminal silicon-methyl (Si—CH3) bonds to more hydrophilic silicon hydroxide (Si—OH) groups. These transformations result in decreased wetting contact angle, higher moisture uptake, increased k-value, and worse leakage current.
To prevent Cu migration into the surrounding dielectric material, the Cu is encased in a barrier layer. The increase in moisture build-up can promote the oxidation of the barrier layer and Cu at the barrier/Cu interface. Such process is known to decrease TDDB lifetime.
Porous low k dielectrics are even more sensitive to moisture uptake hence there is an increased concern on their dielectric breakdown performance.
In summary, some of the factors affecting device reliability are the nature of the Cu/capping layer interfaces, surface contamination of the dielectric materials, and low k dielectric degradation caused by mechanical damage, surface modification from hydrophobic to hydrophilic, and moisture uptake.
While there have been many attempts to improve device reliability by applying metal/metal alloy capping layer on copper interconnect, solutions to the problems related to interface reliability problems have been long sought, but have long eluded those skilled in the art.